1. Field of the Invention
The present invention relates to (Ba,Sr)TiO3 (BST) thin films, and more particularly to a method for creating a haze-free BST thin film with a high dielectric constant.
2. Description of the Related Art
(Ba,Sr)TiO3 (BST) films are commonly used as dielectric materials for capacitors, gate dielectrics and high frequency electronic circuits. More particularly, BST films have found application as capacitors in dynamic random access memory (DRAM) cells. A typical DRAM cell comprises a charge storage capacitor (or cell capacitor) coupled to an access device such as a metal-oxide semiconductor field effect transistor (MOSFET). The MOSFET functions to apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is determined by the capacitance, C=εε0A/d, where ε is the dielectric constant of the capacitor dielectric, ε0 is the vacuum permittivity, A is the electrode (or storage node) area, and d is the interelectrode spacing. The conditions of DRAM operation such as operating voltage, leakage rate and refresh rate, will in general mandate that a certain minimum charge be stored by the capacitor.
BST is desirable for such applications because of its high dielectric constant, low DC leakage, low dispersion up to high frequencies and stable operation at high temperatures. The high dielectric constant of BST thereby gives the material the ability to yield high capacitance when placed between a pair of electrodes. BST films grown for applications such as DRAM capacitors are typically made using metal-organic chemical vapor deposition (MOCVD) or sputtering. However, MOCVD growth of such films typically leads to problems such as haze which reduces the dielectric constant of the material and increases leakage currents. Specifically, haze is caused by the growth of spatially correlated, non-textured BST, which in turn creates discernible optical scatter and a cloudy or hazy appearance in the film. For example, haze may be created when a film desired to be grown in a (100) orientation has orientations other than (100), such as (110) or (111), thereby disrupting the texture of the film. When BST is used in a capacitor structure, haze causes its capacitance to decrease as much as 50% and leakage currents to increase by a factor of 10 to 1000 with respect to smooth films.
Furthermore, electrodes and other materials on which BST films are deposited often suffer from process-induced defects such as hillock formation which may severely limit performance. Hillocks are small nodules which form when the electrode or other material is deposited or subjected to post-deposition processing. For example, hillocks can result from excessive compressive stress induced by the difference in thermal expansion coefficient between the BST film and the underlying electrode material during post-deposition heating steps. Such thermal processing is typical in the course of semiconductor fabrication. Hillock formation may create troughs, breaks, voids and spikes along the electrode surface, thereby leading to uneven BST growth and stress in the BST film.